Trivia: In the scene where Doctor Spivey is interviewing McMurphy, this whole scene is improvised. Spivey (Dr. Dean Brooks) is the ACTUAL doctor of the institute in real life, and was simply told to interview Jack Nicholson (McMurphy) as if he was a real patient. Nicholson had to improvise and get from the beginning of the scene to the end.
Trivia: To further immerse the actors in their environment, director Milos Forman led them in unscripted group therapy sessions in which he directed the actors to develop their characters' mental illnesses organically. He would often capture footage of the actors, both in and out of character, without telling them that the cameras were rolling. The film's final cut includes a shot of a visibly irritated Fletcher reacting to a piece of direction fed to her by Forman.
Trivia: All of the actors who portrayed patients in the film actually lived on-site in the the Oregon State Hospital psychiatric ward during the production. The men personalized their sleeping quarters, interacted with real psychiatric patients and, as actor Vincent Schiavelli put it, spent part of their days getting a sense of what it was to be hospitalized.